Turbo machine airfoils can be subject to high static and dynamic loads due to thermal and centrifugal loads as well as dynamic excitation forces. The resulting vibration amplitudes, in combination with the high static loads, can lead to high cycle fatigue failures. Thus, the damping of vibration can be of great importance.
One way to address this problem is to install frictional coupling devices, such as under platform-dampers, lacing wires or tip shrouds that provide damping through energy dissipation by frictional contact. This approach can be disadvantageous due to design complexity because physical contact parameters can be difficult to evaluate and change under operating conditions. Furthermore, the coupling of the airfoils and the geometric properties of friction damping devices can change dynamic characteristics such as eigenfrequency and mode shape.
An alternative can be to use the attractive force of magnets for damping. U.S. Pat. No. 4,722,668, for example, discloses the use of magnets in both the shroud and at half airfoil height. The magnets are paired, so that the magnet of one airfoil abuts a magnet fitted in an adjacent airfoil.
As an alternative, eddy currents induced by movement of an electrical conductor in a magnetic field can provide an alternative with a different damping capability. This solution uses the principle that the movement of an electrical conductor in a magnetic field induces a voltage, which in turn creates eddy currents. The magnetic field of the eddy currents opposes that of the first magnetic field. This exerts a force on a metal plate causing it to resist movement while transforming kinetic energy of a conductor plate into heat.
DE 195 05 389 A1 for example, discloses an eddy current damping arrangement for a turbo machine in which a magnetic ring is located in a wall of a turbo-machine such that the vibration of rotating airfoils, which are equipped with an electric conductor, can be suppressed when passing the ring.
U.S. Pat. No. 7,399,158 B2 discloses another eddy current damping system applied to an array of airfoils mounted for rotation about a central axis. The damping arrangement includes a current carrying conductor that can form a loop around the array of airfoils.
Both of these arrangements involve the installation of a magnetic ring, or ring shaped current carrying loop for inducing a magnetic field, that is separate from the airfoils. As an alternative, DE 199 37 146 A1 discloses adjacent airfoils with paired wings having ends in close proximity to each other. The end of one wing has a mounted magnet while the end of its paired opposite has a copper or aluminium plate. By these features the relative movement of the wing end can be suppressed by the eddy current principle.
Unlike vibration suppression systems that use magnetic attraction, vibration damping by eddy currents involves some relative movement without which eddy currents will not be formed. All of the foregoing documents are incorporated herein by reference in their entireties.